Somethine to try
I have a BFG 8800GTSOC 640MB card and I'm using RivaTuner 2.01. I have my 2d clocks set to 470 and my 3D set to 580 and yes, they do work. My fan speed is at 100%. Idle temps are down to 51C, under load they hit 63C Ambient temp is 38C. So if you haven't tried setting 2d Clocks using riva tuner with your 8800 card, give it try, it does work and the 3D clocks will kick in when you play a 3D app.
Xion, I dont know if RT works with the 2900, but if it does, wanna see if you can set seperate 2D and 3D clocks with your card? Post back with result if it works.